1. 'Umar, radiallaahu' anhu reports that the Prophet said, "Stick to the Jamaa'ah and beware of splitting, for Shaitaan is found along with the single person but is further from two. Whoever desires the centre of Paradise then let him stick to the Jamaa'ah. He whose good deed pleases him and his evil deed causes him to feel bad then he is a believer." Reported by Ahmad (1/18), at-Tirmidhee (no. 2165) al-Haakim (1/114) and is declared saheeh by Shaikh al-Albaanee in as-Saheehah (no. 1116).
Anas, radiallaahu 'anhu, reports that the Prophet said, "Indeed the Children of Israa'eel divided into seventy one sects and my Ummah will divide into seventy two sects, all of them will be in the fire except one and that is the Jamaa'ah." Reported by Ibn Maajah (no. 3993) and declared saheeh by Shaikh al-Albaanee.
2. Alluding to the hadith of Ibn 'Abbaas, radiallaahu 'anhummaa, who said: Allaah's Messenger said,"He who sees from his ruler something he dislikes, let him be patient with him, for he who splits away from the Jamaa'ah by a handspan and then dies, dies a death of Jaahiliyyah" and in a narration, "then he has thrown off the yoke of Islaam from his neck." Reported by al-Bukhaaree (trans. 9/145), Muslim and Ahmad and the other narration by at-Tirmidhee (no. 2867) and Ahmad (4/130). Refer to Sharh ul-'Aqeedat-Tahaawiyyah (379-382), checked by Shaikh Naasir ud-deen al-Albaanee.